Phase Control Block for Managing Multiple Clock Domains in Systems with Frequency Offsets

ABSTRACT

A circuit for performing clock recovery according to a received digital signal  30.  The circuit includes at least an edge sampler  105  and a data sampler  145  for sampling the digital signal, and a clock signal supply circuit. The clock signal supply circuit provides edge clock  25  and data clock  20  signals offset in phase from one another to the respective clock inputs of the edge sampler  105  and the data sampler  145.  The clock signal supply circuit is operable to selectively vary a phase offset between the edge and data clock signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/820,266, filed Aug. 6, 2015, which is continuation of U.S.application Ser. No. 14/321,723, filed Jul. 1, 2014, now U.S. Pat. No.9,106,399, which is a continuation of U.S. application Ser. No.13/710,404, filed Dec. 10, 2012, now U.S. Pat. No. 8,774,337, which is acontinuation of U.S. patent application Ser. No. 12/225,999, filed Jan.12, 2009, now U.S. Pat. No. 8,331,512, which is a United States Nationalstage application filed under 35 U.S.C. §371 of PCT Patent ApplicationSerial No. PCT/US2007/008493, filed Apr. 4, 2007, which claims thebenefit of the filing date of U.S. Provisional Patent Application No.60/789,406 filed Apr. 4, 2006, all of which are incorporated herein byreference in their entireties.

BACKGROUND

The present invention relates to clock recovery circuits and relatedcircuits used for receiving and transmitting digital signals.

Modern digital systems typically include multiple digital devicescapable of communicating with each other using digital signals made ofsequences of digital symbols. A digital symbol takes up a time interval,which is often referred to as a unit interval or a data interval. Adigital device may transmit a digital data signal by setting the valueof a signal parameter associated with a communication channel to one ofa plurality of predetermined values for each data interval according toa transmitter clock. A digital device that receives the digital signaltakes successive samples of the signal according to a recovered clockand determines the values of the signal parameter represented by thesamples. The recovered clock is preferably synchronized with the digitalsignal so that the samples are taken at times corresponding to the dataintervals.

To communicate data at high speed, it is desirable to make the dataintervals as short as possible, which, in turn, requires greaterprecision in the synchronization between the recovered clock used by thereceiving device and the data intervals of the incoming signal. Forexample, modern interfacing and communication protocols such as PCIExpress, SONET, InfiniBand and XAUI use data intervals on the order ofnanoseconds or less, and require that the receiving device use a clockwhich is synchronized to the data intervals to within a fraction of onedata interval.

In a typical link between two digital devices a receiving device employstwo clock domains to sample a received digital signal. The two clockdomains employed are the edge clock domain and the data clock domain.The edge clock domain is used to sample the digital signal at or nearthe boundaries of the data intervals. The data clock domain is used tosample the digital signal at a point between the boundaries of the dataintervals so as to minimize the effect that signal transitions may haveon the data domain samples.

The edge and data domain samples of the digital signal are used tosynchronize the recovered clock to the transmitter clock. Morespecifically, the edge and data domain samples are supplied to a phasedetector within the receiver which, in turn, uses the edge and datadomain samples to generate a phase error signal indicative of therelative phase error between the transmitter clock and the recoveredclock. The phase error signal is passed to a phase controller within thereceiver, and the phase controller adjusts the phase of the edge anddata clocks in accordance with the phase error signal. Thus, the phasedetector and phase controller are part of a feedback loop that serves tominimize the phase error signal.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The following detailed description given by way of example, but notintended to limit the invention solely to the specific embodimentsdescribed, may best be understood in conjunction with the accompanyingdrawings wherein like reference numerals denote like elements and parts,in which:

FIG. 1 is a block diagram of a clock and data recovery circuit inaccordance with a preferred embodiment.

FIG. 2 is a block diagram of a phase detector in accordance with apreferred embodiment.

FIGS. 3A-3D are timing diagrams useful for understanding the operationof the phase detector shown in FIG. 2.

DETAILED DESCRIPTION

It has been recognized that it is desirable to allow for a selectivelyvariable phase offset between clock domains. Further, it has beenrecognized that it is desirable for digital receivers to employ three ormore clock domains, and that there is a need for systems and methods tosynchronize such multiple clock domains with a transmitter clock. Stillfurther, it has been recognized that it is desirable to provide forselectively variable phase offset and/or the synchronization of three ormore clock domains.

For purposes of clarity of presentation, the preferred embodiments willbe discussed in the context of second order clock recovery. Upon reviewof this disclosure, one skilled in the art will readily appreciate howthe present system and method is applied in the context of first orderclock recovery and higher order clock recovery.

A circuit according to one embodiment is shown in FIG. 1. The circuit ofFIG. 1 includes a feedback circuit 10 arranged to receive a phase errorsignal 15 indicating a relative phase relationship between an incomingdigital signal 30 and a data clock signal 20 and/or edge clock signal25. The phase error signal is supplied to a first proportionalityconstant unit 35 (“Kp”) which multiplies the phase error signal by aproportionality constant to provide a first signal component 40 directlyrelated to the phase error. The phase error signal is also supplied to asecond proportionality constant unit 45 (“Ki”) which multiplies thephase error signal by a second proportionality constant and supplies theresulting signal to an accumulator 50 (“Freq. ACC”) which in turnprovides an output or second signal component 55. When the system is inlock with a digital signal 30, this output represents the frequencyoffset between the Receiver clocks and the digital signal 30. An adder60 combines the first and second signal components to provide a phaseadjustment signal 65 which includes both of these components. The phaseadjustment signal may be positive or negative depending on the sign ofthe phase error and its integral. Although FIG. 1 depicts threeidentical adders 60, 70, and 75, providing three identical phaseadjustment signals, a single adder may be used. The feedback circuit isa “second order” circuit.

The phase adjustment signal is supplied to three accumulating registers80, 85, and 90 (“Phase ACC Edge,” “Phase ACC DFE,” and “Phase ACC TX”).Each register maintains a phase shift value and increments or decrementsthe phase shift value in response to the phase adjustment signal.

One register (“Phase ACC Edge”) is connected directly to a control input92 of an edge clock phase shifter 95 (“Interpolator Edge”) which in thisembodiment is a phase interpolator. Other types of phase shifters may beused in place of each interpolator referred to herein as, for example,adjustable delay lines or combinations of delay lines and phaseinterpolators. The edge clock phase shifter is arranged to receive aclock signal 100 referred to herein as the “Receiver clock signal.” TheReceiver clock signal may be generated by a phase locked loop (notshown) or other conventional circuit and in many embodiments will bemultiple clock signals of the same frequency but equally spaced within aperiod of the Receiver clock signal. The edge clock phase shifter shiftsthe phase of the Receiver clock signal by an amount directly related tothe output of Phase ACC Edge. The phase-shifted replica of the Receiverclock signal constitutes the edge clock signal. The edge clock signal isapplied to the clock input of a latch or other sampling device 105(“Edge Sampler”). This sampling device receives a digital signal 110(“equalized signal”), captures successive samples of the digital signalat times set by the edge clock signal and compares these samples to athreshold to convert each sample to “edge samples” 115, which are in theform of a digital “1” or “0.” These “edge samples” are supplied to adeserializer and phase detector 120.

A data phase shifter 125 (“Interpolator Data”) has a control input 130connected to an adder 135 which receives the phase shift value from thesame register (“Phase ACC Edge”) and which also receives a data clockoffset value 140 (“D_os”) which in this embodiment is selectable by aninput applied to the circuit. For example, D_os may be a value held in auser-programmable register or a value supplied by another circuit. Thus,the value supplied to the control input of the data phase shifter willbe the sum of the phase shift value stored in register Phase ACC Edgeand the data clock offset value. The data phase shifter also receivesthe Receiver clock signal and produces a replica of this signal (the“data clock” signal 20) phase-shifted by an amount directly related tothis sum. Thus, the phase offset between the edge clock signal and thedata clock signal is selectively variable by varying D_os. The dataclock signal is applied to a data sampler 145 which functions in thesame way as the edge sampler discussed above to sample the digitalsignal (“equalized signal”) provide a series of 1 and 0 values 150(“data samples”) to the deserializer and phase detector. Thedeserializer and phase detector provides data samples 155 as paralleldata bytes and also determines from the values of the data and edgesamples whether the data clock and edge clock are early or late relativethe timing of the digital signal. The deserializer and phase detectorthus provides the phase error signal in the form of a signal indicatingwhether the data and edge clocks are early or late relative to thedigital signal. In this embodiment, the phase detector is integratedwith the deserializer. One form of such a phase detector is shown inFIG. 2.

FIG. 2 shows a preferred embodiment of deserializer and phase detector120′ along with associated data sampling circuitry 500. Thedeserializing portion of the deserializer and phase detector isindicated generally as element 122. The phased detecting portion of thedeserializer and phase detector is indicated generally as element 126.

It should be noted that deserializer and phase detector 120′ may havetwo more inputs than deserializer and phase detector 120 of FIG. 1,delayed data samples 505 and delayed edge samples 510. Thus, the datasampling circuitry of FIG. 2 includes data sampler 734 and edge sampler736 for delayed sampling of the equalized signal.

It should be further noted that delayed sampling is well known in theart, and that upon review of this disclosure one skilled in the art willreadily understand how the invention is implemented in the context ofdelayed sampling and the deserializer and phase detector of FIG. 2.

In any event, samplers 145, 105, 734, and 736 are in the form oflatches. The samplers are respectively clocked by data clock 20, edgeclock 25, a delayed data clock 515, and a delayed edge clock 520. Thedelayed data clock has the same frequency as the data clock but isdelayed in phase by 180 degrees relative to the data clock. Similarly,the delayed edge clock has the same frequency as the edge clock but isdelayed in phase by 180 degrees relative to the edge clock. Each of thedata clock, edge clock, delayed data clock, and delayed edge clock has aperiod equal to two data intervals.

In the ideal locked condition shown in FIG. 3A, the data clock anddelayed data clocks are aligned in the centers of successive dataintervals, whereas the edge clock and delayed edge clock are alignedwith the boundary or edges between successive data intervals. The serialdata input is connected to latch 145, clocked by clocked by the dataclock; to latch 105, clocked by the edge clock; to latch 734 clocked bythe delayed data clock; and to latch 736, clocked by the delayed edgeclock. Accordingly, in the locked condition shown in FIG. 13A, latches145 and 734 will sample the serial data signal in alternate dataintervals, referred to herein as “even” and “odd” data intervals,whereas latches 105 and 736 will sample the serial data signal atalternate even and odd boundaries between data intervals. Thus, the dataclock and delayed data clock can be referred to as even and odd dataclocks, whereas the edge clock and delayed edge clock can be referred toas even and odd edge clocks.

In this manner, the samplers output data samples 150, edge samples 115,delayed data samples 505, and delayed edge samples 510. The samplesoutput from the samplers are passed to deserializer and phase detector120′. The deserializer and phase detector 120′ includes a digital phasedetector 122 and a digital loop filter 126.

The outputs of latches 145 and 734, clocked by the data clocks, areconnected through a multiplexer 738 to the input of a shift register740, referred to herein as the data register. Thus, as the latches andmultiplexer operate, 1 or 0 values from latches 145 and 734,representing samples taken during successive even and odd dataintervals, will be clocked into register 740. The outputs of latches 105and 736, clocked by the edge clocks, are connected through anothermultiplexer 742 to a shift register 744, referred to herein as the edgeregister. Each shift register is arranged to hold n bits, where n isequal to the number of bits in a byte of parallel data. Thus, after ndata intervals, data register 740 will hold data as shown in FIG. 2,with an even bit D0 representing the sample taken during a first, evendata interval in the first position, an odd bit D1 representing thesample D1 taken during the next, odd data interval, and so on.Similarly, the first position in edge register 744 will hold a first,even bit E0 representing the sample taken at the boundary between thefirst and second data intervals, i.e., at the boundary between the dataintervals represented by bits D0 and D1. Likewise, the second positionin edge register 744 will hold an odd bit E1 representing the sampletaken at the boundary between the data intervals represented by bits D1and D2, and so on.

Data register 740 is arranged to supply all of the bits together, as theparallel data output of the deserializer. The data and edge registers740 and 744 also are connected to a logic circuit 746. Logic circuit 746is arranged to perform an exclusive or (XOR) operation between each databit in data register 740 and the next succeeding bit in the dataregister to derive a transition detect signal. The logic circuit 746 isalso arranged to perform an XOR operation between each data bit in dataregister 740 and the corresponding edge bit in register 744 to providean early/late signal. For example, the XOR of D0 and D1 provides atransition detect signal associated with D0, whereas the XOR of D0 andE0 provides an early/late signal associated with D0. Logic circuit 746is arranged to provide a count value for each byte equal to the numberof early/late signals for that byte having value 1 minus the number ofearly/late signals which have value 0. However, the logic circuit isarranged to exclude from the count the early/late signal associated witheach data bit if the transition detect signal associated with that databit is 0. A positive number indicates that the clock is late relative tothe data signal, whereas a negative number indicates that the clock isearly relative to the data signal.

The operation of the digital phase detector of FIG. 2 can be betterunderstood with reference to FIGS. 3A-3D. In a perfectly lockedcondition (FIG. 3A), each edge sample represented by the bits in theedge register is taken precisely at the boundary between data intervals.The voltage used to represent digital 1 or 0 has an indeterminate value,somewhere between the high value representing 1 and the low valuerepresenting 0. Thus, when latch 105 or latch 736 samples the signal,the probability the digital value output by the latch will be 1 is equalto the probability that the value will be 0. Therefore, the number of 0early/late signals generated by the logic circuit will be equal to thenumber of 1 early/late signals.

Where the clock is early (FIG. 3B), each edge bit represents a sampletaken during the immediately preceding data interval, rather than at theboundary or edge between data intervals. Therefore, each edge bit inregister 744 will have the same value as the corresponding data bit inregister 740, and hence each early/late signal will be 0. Where theclock is late (FIG. 3C), each edge bit represents a sample taken duringthe immediately succeeding data interval, rather than at the boundarybetween data intervals. If there is a transition in the data betweenthese intervals, the edge bit will have a value different from thecorresponding data bit, and the early/late signal will be 1.

If there is no transition between two successive data intervals, so thatthe data bits are the same, the edge bit will have the same value as thedata bits regardless of whether the clock is early or late, and theearly/late signal will be 0. However, in this case, the transitionsignal will also be 0 and the early/late signal is ignored.

The count from logic circuit 746 is supplied to a scaling factor unitwhich supplies a value equal to the value from register 718 multipliedby a scaling factor, and to an integrator 712 which integrates the valuesupplied by register 708 over time and applies an appropriate scalingfactor. The output of integrator 712 and scaling factor unit 710 areperiodically sampled by a combining circuit 714. The combined value fromunit 714, thus, represents a combination of a first-order signal fromscaling unit 710 representing substantially instantaneous clock signallead or lag, and a second-order component from integrator 712representing the integral of the lead or lag over time. Combining unit714 supplies each such combined value to a dumped integrator 716 linkedto a threshold detection unit 718, which, in turn, is linked to a barrelcounter 720. Integrator 716 accumulates a total representing all of thesignals from combining unit 714. If the total reaches a positivethreshold, threshold detection unit 718 issues a count-up signal tobarrel counter 720 and a dump signal to integrator 716, which resets theintegrator to 0. If the total reaches a negative threshold, thethreshold detection unit 718 issues a count-down signal to barrelcounter 720 and also resets the integrator 716 to 0. Counter 720 holds acount value; it increments the count by a preselected increment for eachcount-up signal and decrements the count by the same increment for eachcount-down signal. Counter 720 counts along a circular scalecorresponding to 360 degrees. Thus, assuming that the barrel counter isoperating with a 1 degree increment and the current count is 359degrees, the count will be reset to 0 if a count-up signal is received.Similarly, if the count is currently 0 degrees, the count will be resetto 359 degrees if a count-down signal is received. The output of thebarrel counter constitutes the digital control signal.

The particular implementation shown in FIG. 2 can be varied. Forexample, combiner 714 can be replaced by a multiplexer which can beactuated to select either the second order signal from integrator 712 orthe first order signal from scale factor 710, so as to provide only afirst-order characteristic or only a second-order characteristic. Also,the scale factor unit 710 may be omitted entirely if only a second-ordercharacteristic is desired, and integrator 712 may be omitted if only afirst-order characteristic is desired. The components of the digitalphase detector may be replaced by other circuits which perform theequivalent operation of edge detecting the data signal and multiplyingthe clock signal by the edge detection signal. See “MonolithicPhase-Locked. Loops And Clock Recovery Circuits—Theory And Design,”Razavi, ed., February 1996, pp. 33-35. The digital loop filter willretain a fixed value for the digital control signal, even during aprolonged absence of transitions in the data signal.

Other types of phase detectors can be used as well. In any case, thedata and edge clocks thus operate in conjunction with one another on thereceived signal. The term “recovered clock” refers to either or both ofthese clock signals.

The ability to vary the offset between the data clock and edge clock canbe used, for example, to minimize the bit error rate where the digitalsignal has an asymmetric data eye.

An adaptive clock phase shifter 160 (“Interpolator Adapt”) is connectedto an adder 165 which receives the phase shift value from the sameregister (“Phase ACC Edge”) and also receives a selectively variableadaptive clock offset value 170 (“A_os”). The adaptive clock phaseshifter provides another replica (“adaptive clock”) 22 of the Receiverclock, phase shifted by an amount directly related to the sum of thephase shift value stored in register Phase ACC Edge and A_os, to afurther sampler 175 which also samples the equalized signal. Theselectively variable offset between the adaptive clock and the data andedge clocks allows control of the adaptive sampler to take samples atany desired point on the digital signal waveform. This capability can beused to measure the size of the data eye of the digital signal while thesystem continues to collect the data using the data clock and edge clockas discussed above. Furthermore, the values provided by the adaptivesampler are provided to a circuit 180 (“adaptation”) which may alsoreceive the values from the data sampler. The “adaptation” block can usethese sampler outputs to optimize the equalizers “DFE” to obtain abetter BER performance through the link. That the process of adapting anequalizer requires these samples is obvious to those skilled in the art.

In the embodiment discussed above, the edge phase shifter (“InterpolatorEdge”) is connected directly to the register (Phase ACC Edge”), whereasthe data and adaptive phase shifters are connected to the register viathe adders. The reverse arrangement, with the data phase shifterdirectly connected and the edge clock connected to an adder for adding aselectively variable offset, can be used.

A decision feedback equalization or “DFE” circuit output 185 isconnected to a summing node 191 which accepts the received digitalsignal 30. The DFE circuit supplies a signal which depends on the valuesof the immediately preceding bits in the received signal, i.e., on thedata values derived during immediately preceding bit intervals, so as toproduce the equalized signal discussed above. The DFE circuitcompensates for inter-symbol interference. DFE circuits per se are knownand are not further described herein. However, in FIG. 1, the timing ofthe DFE circuit is controlled by a DFE clock 190 which is provided by aDFE phase shifter 195 (“Interpolator DFE”). The DFE phase shifterprovides a further replica of the Receiver clock signal, phase shiftedby an amount which is controlled by the sum of the phase shift value inregister Phase ACC DFE and a selectively variable offset 200 (DFE_os).The sum of the phase shift value in register Phase ACC DFE and theselectively variable offset 200 is formed by an adder 205. The abilityto vary the offset allows optimization of the timing of the DFE circuit.

A transmitter 210 (“TX”) transmits data values 215 supplied to thetransmitter using a clock signal 220 (“Transmit Clock”). The transmitclock signal is derived by a further phase shifter 225 (“interpolatorTX”) as a replica of the Receiver clock signal phase shifted by the sumof a selectively variable offset 230 (“TX_os”) and a phase shift valuestored in register Phase ACC TX. The sum of selectively variable offset230 and the phase shift value stored in register Phase ACC TX is formedby an adder 235.

While in normal operation the Phase ACC TX is disabled so that theTransmit Clock is stationary, it is advantageous for link diagnosticpurposes for it to track the movement of the recovered clock with afixed phase offset. For instance, by setting the transmit data to be aseries of alternating 1's and 0's while enabling the Phase ACC TX, thetransmitted signal becomes a clock signal which has a fixed phaserelationship to the edge clock. This allows a method by which thevariations on the phase of the recovered clock can be observed withoutrequiring separate pins or probing even in the presence of frequencyoffsets. During normal use of the chip, the transmitted signal is usedto convey real data while the register Phase ACC TX is disabled by asignal 240 (“En_TX”). Similarly, the linkage between the DFE signal andthe other clocks can be enabled or disabled.

Components 245, 250, and 255 of FIG. 1, respectively marked “BinaryCony,” “−1” and “Invert,” can be selectively actuated to lock the phaseerror signal at zero or to invert the phase error signal.

In a variant of the embodiment discussed above, one or both of the DFEclock and transmit clock can be derived from the phase shift valuestored in the same register used for the other clock signals (“Phase ACCEdge”). In a further variant, some of the features discussed above canbe omitted. For example, the system can be used without the transmitterand transmit clock, or without the adaptive clock. In another example,the relationship between the data clock and edge clock can be fixed. Ina further variant, the offset values can be derived automatically duringoperation. In yet another variant, the feedback circuit may be afirst-order or higher-order circuit.

The particular embodiments shown above are merely illustrative. Forexample, the circuitry discussed above can be implemented in any desiredform as, for example, as TTL or CMOS circuitry. The invention can beapplied to data signals other than electrical signals as, for example,optical data signals. Also, the invention can be applied to data signalsother than binary signals as, for example, in multi-level signaling,also referred to as pulse amplitude modulation signaling. For example,in a PAM 3 signal, the value or characteristic of the data signal mayhave any one of three values during each data interval. In theembodiments discussed above, the data signal is a signal sent by asending device outside of the monolithic integrated circuit whichincorporates the clock recovery circuit. However, the same clockrecovery circuit can be employed where the data signal is sent fromanother portion of the same monolithic integrated circuit.

As these and other variations and combinations of the features discussedabove can be utilized without departing from the present invention asdefined by the claims, the foregoing description of the one embodimentsshould be taken by way of illustration rather than by way of limitationof the invention as defined by the claims.

1. A circuit for receiving a digital signal comprising: at least threesamplers for sampling the digital signal, each sampler having a clockinput, wherein the samplers include an edge sampler and a data samplerand an adaptive sampler; and a clock signal supply circuit for providingrespective clock signals to respective clock inputs of the samplers,wherein each pair of all possible pairs of said clock signals has arelative phase between the two clock signals making up the pair, andwherein the clock signal supply circuit is operative to selectively varythe relative phase for a first pair of said clock signals and toselectively vary the relative phase for a second pair of said clocksignals.